The term "used industrial oil" is used herein to mean used industrial oils which are blended to specific requirements for use in non-motor vehicle applications in industrial or power producing plants. This term does not, however, mean used crank case oil from motor vehicles such as, for example, cars, trucks and locomotives, as well as gear oils, automatic transmission fluids and other functional fluids in which the major constituent is an oil of lubricating viscosity.
Most existing reclaiming plants for rerefining oil use sulfuric acid to coagulate as an acid sludge for ash and polar components in used oil. This procedure, followed by treatment with alkaline solutions to neutralize the acid, water washing, active clay decolorizing, stripping and filtration yields a lube stock suited to reuse as a low grade motor oil or as a grease base. The poor yield of rerefined oil and environmental problems of disposal of acid sludge and clay make such a reclaiming process a marginal operation at best.
Various alternative approaches have been proposed for reclaiming used oil. Propane extraction prior to acid treatment has been reported as reducing the amount of acid and clay required, but the yield of recovered oil remains at only about 65% and plant investment costs are much higher. Vacuum distillation has been suggested and work has been done on hydrotreating of distilled oil to lube stock. This latter process leaves a high ash residue and serious problems in fouling of heat exchanger and fractionation equipment have been encountered. Solvent extraction process have been proposed for reclaiming used lubricating oils, but the volume of solvent required has generally been at least equal to the volume of oil being treated and more often at least two to three times the volume of such oil, thus leading to high equipment costs and solvent recovery problems.
U.S. Pat. No. 3,919,076 describes a process for rerefining used automotive lubricating oil that includes the steps of first purifying the oil of debris, dehydrating the oil, then mixing the oil with 1-15 times the volume of such oil of a solvent selected from the group consisting of ethane, propane, butane, pentane, hexane and mixtures thereof, the preferred solvent being propane. The patentee indicates that a special scrubber is used to remove heavy metal particulates from the combustion gases and then the oil-solvent mix is stripped, subjected to vacuum distillation, hydrogenation, another stripping process and filtering.
U.S. Pat. No. 3,930,988 describes a process for reclaiming used motor oil by a series of treatments of such oil that includes mixing the oil with ammonium sulfate and/or ammonium bisulfate under conditions that react the sulfate or bisulfate with metal-containing compounds present in the used oil to precipitate contaminants from the oil. The patentee indicates that an optional step of further treating the oil under hydrogenation conditions can be employed to remove additional contaminants and produce a low ash oil product.
U.S. Pat. No. 4,021,333 describes a process for rerefining oil by the steps of distilling used oil to remove a forecut having a viscosity substantially less than that of lubricating oil, continuing the distillation to recover a distillate having substantially the viscosity of lubricating oil, extracting impurities from the distillate of the foregoing step with an organic liquid extractant, and removing the organic liquid and impurities dissolved therein from the distillate.
U.S. Pat. No. 4,028,226 describes a process for rerefining used oil by the steps of diluting the used oil with a water-soluble polar diluent, removing a major amount of the polar diluent from the solution by addition of water and removal of the resulting aqueous phase, and removing the balance of the polar diluent from the oil. The patentee indicates that useful diluents are the lower alkanols and lower alkanones.
U.S. Pat. Nos. 4,073,719 and 4,073,720 describe methods for reclaiming used oil that include the use of a solvent for dissolving the oil and precipitating metal compounds and oxidation products from the oil as sludge. The solvent that is described as being preferred consists of a mixture of isopropyl alcohol, methylethyl ketone and n-butyl alcohol. The solvent-to-used-lubricating-oil ratio is indicated to be in the range of about 8 to about 3 parts solvent to one part oil.
It would be advantageous to provide a process for rerefining used industrial oil in a manner resulting in a relatively high yield and relatively small quantities of sludge and other undesirable waste products. It would be advantageous if such sludge and waste products could be collected in such a manner so as to be incinerated to provide a heat source for power generation. Finally, it would be advantageous if the final product produced from such a process exhibited properties comparable to virgin oil.